Is this gps driven? – ProximiT: MBTA Boston Transit Review

I’ve used it for 12 weeks and most of the time it’s spot on but occasionally it shows a bus arriving in 1 min and then all of a sudden it jumps to 15 min, and then a bus actually arrives in 4 min. This make me suspicious it is primarily driven by MBTA schedule and not real bus gps. Also please add a ‘leave now’ timer that I can set for X minutes before my bus arrives
Review by ECFL on ProximiT: MBTA Boston Transit.